Integrated circuits (ICs) often require selective one time programmable (OTP) permanent electrical connections between circuit nodes. Such a connection can be implemented by an antifuse. Antifuses are often used to permanently store binary data on an IC. Binary logic states are represented by "on" and "off" states of the antifuse. Antifuses are used in numerous memory storage applications including programmable logic arrays (PALs), programmable logic devices, and programmable read only memories (PROMs). Antifuses are also often used in memory cell arrays such as dynamic random access memories (DRAMs). After testing the DRAM for failing memory cells, failing cell addresses in the DRAM are remapped to functional cell addresses by selective permanent programming of antifuse elements.
Antifuses are fabricated with structure similar to that of a capacitor; two conductive electrical terminals are separated by a dielectric layer. An unprogrammed "off" state, in which the antifuse is fabricated, presents a high resistance between the antifuse terminals. The antifuse can also be programmed to an "on" state in which a low resistance connection between the antifuse terminals is desired. To program an antifuse "on," a large programming voltage is applied across the antifuse terminals, breaking down the interposed dielectric and forming a conductive link between the antifuse terminals.
Once an antifuse has been programmed, it permanently retains its programmed state. Correctly detecting the programmed state of an antifuse is complicated by variations in the fabrication process, inherent time delay, and degradation of the antifuse dielectric over time. Typical antifuses are implemented as relatively large elements to offset these and other complications. Large antifuses are more robust and less prone to degradation over time.
However, as integrated circuit technology advances, the size of individual circuit elements decreases. Thus, designers can include more storage cells in a memory array on a semiconductor substrate. As the number of storage cells increases, the number of antifuses needed also increases. Unfortunately, conventional antifuses are too large to fulfill the growing need for more antifuses in memory devices.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for antifuse elements that conserve semiconductor substrate surface area while maintaining high reliability characteristics.